The present invention is directed to the detection of supervisory audio tones and other signaling tones in cellular-telephone signals. In particular, it is directed to detecting these signals digitally.
It has become feasible to perform almost all cellular-system signal processing digitally. In a cellular-telephone base station, for instance, initial analog processing may be limited to translating a multi-channel portion of the cellular-telephone spectrum to an intermediate frequency, and analog-to-digital conversion is performed on the resultant intermediate-frequency signal. All subsequent processing can be performed digitally.
The individual channel typically comprises a frequency-modulated signal that spans a nominally 30-kHz bandwidth. To represent the channel's contents digitally, therefore, one may employ, say, complex samples that occur at a 40 kHz rate and can be demodulated to a 40-kHz real-valued stream. The resultant, demodulated signal's bandwidth typically is great enough to include not only voice but also supervisory tones, whose frequencies are above the range employed for the voice signal.
These tones must be discriminated with relatively great frequency resolution from other signal components. In the AMPS standard, tones of 5970 Hz, 6000 Hz, 6030 Hz, and 10,000 Hz must be distinguished from each other and from noise. Of course, high frequency resolution requires processing relatively long-duration signal records, and this combination of long duration and wide spectrum tends to impose a heavy storage and computational burden.